An electrophotographic lithographic printing master plate has a photosensitive layer composed of a binder resin having dispersed therein a photoconductive fine powder such as a metal oxide (e.g., zinc oxide, titanium oxide, etc.) and a metal sulfide (e.g., zinc sulfide, etc.) and printing images are obtained by applying an ordinary electrophotographic processing onto the layer to form ink-receptive images thereon.
In general, for offset printing, a printing plate composed of a non-image area (hydrophilic area) which is liable to be wetted with water and an image area (ink-receptive area) which is reluctant to be wetted with water is used but since an electrophotographic offset printing master plate is composed of a hydrophobic photoconductive layer, if printing is applied using the printing master plate as it is, a printing ink also attaches to the non-image area and thus normal printing cannot be performed.
Accordingly, it is necessary to subjecting the non-image area to a desensitizing treatment prior to printing to render the non-image area hydrophilic. Hitherto, as the desensitizing treatment liquid, a cyan compound-containing treatment liquid containing a ferrocyanate or a ferricyanate as the main component and a cyan-free treatment liquid containing an ammine cobalt complex, phytic acid (inositol hexaphosphate) and the derivative thereof, or a guanidine derivative as the main component are proposed.
However, these treatment liquids are not said to be the treatment liquids which can be sufficiently satisfactory. That is, in the case of the former desensitizing treatment liquid containing a ferrocyanate or a ferricyanate, there are advantages that the treatment liquid has a strong desensitizing power, has a faculty of forming a strong hydrophilic coated film, and the film-forming speed is high but the former desensitizing treatment liquid has the disadvantages that the ferrocyanate ion or the ferricyanate ion is unstable to light, when the ion is exposed to light, the ion is colored and precipitates form to weaken the desensitizing power, and further since the treatment liquid contains the cyan ion (CN), when free cyan is detected, it causes various problems in the points of environmental pollution such as the waste solution, etc.
On the other hand, by considering the points described above, the latter cyan-free treatment liquid containing an ammine cobalt complex, phytic acid, a guanidine derivative, etc., as the main component is proposed but the cyan-free treatment liquid cannot yet be said to be a treatment liquid of obtaining a sufficiently satisfactory lithographic printing master plate. Practically, the latter treatment liquid has the disadvantages that the film-forming speed by the treatment liquid is slow as compared with the former treatment liquid and in an etching system of using a processor, by passing once a lithographic printing master plate, through the processor, a hydrophilic coated film having a high physical strength capable of immediately printing cannot be formed, and scumming occurs and the halftone dot gradation is plugged.
Hitherto, it is well known that phytic acid and the metal derivative thereof form metal chelate compounds, and phytic acid and various derivatives thereof are proposed as the desensitizing agent for offset printing master plates. However, the desensitizing treatment liquid containing each of these desensitizing agents has as slow film-forming speed and by the one treatment of the processor, a hydrophilic coated film capable of printing is not formed, which cause the disadvantages that the ink separation property is bad and scumming and plugging of the halftone dot gradation occur.
Thus, the investigations of adding various additives to the phytic acid series treatment liquids have been made for solving the problems described above.
Practically, there are the phytic acid series treatment liquid using the metal complex of an amino-carboxylic acid together as described in JP-B-2-39397 (the term “JP-B” as used herein means an “examined Japanese patent publication”), the phytic acid series treatment liquid using a hexametaphosphate together as described in JP-B-62-7597, the phytic acid treatment liquid added with a lower amine, an alkanolamine, a polyamine, etc., as described in JP-A-54-117201, JP-A-53-109701, JP-A-1-25994, etc., (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), etc. These treatment liquids have a good water receptivity at the beginning of use but have the problems that when they are continuously used, etching is lowered and the water receptivity is lowered, and when these treatment liquids are used after storing for a long period of time, the water receptivity is lowered and scumming occurs, whereby the sufficient effects are not obtained in the case of using these treatment liquids.
Moreover, there is the treatment liquid added with a cationic polymer described in JP-A-60-23099 but when the treatment liquid is continuously used or after storing the treatment liquid for a long period of time, the faculty thereof is lowered and a rust forms as in the case of using the treatment liquids described above.
Furthermore, the phytic acid series desensitizing treatment liquid using a polyethyleneimine copolymer together is proposed as described in JP-A-7-68967, JP-A-7-137475, etc.) is proposed but in the treatment liquid, the problems that the latitude of both the formation of the hydrophilic property at the non-image area by etching and the formation of the ink-attaching property to the image area is narrow, when the treatment liquid is continuously used for a long period of time, the faculty is lowered, etc., have not yet been solved.
On the other hand, recently, from the view point of work-saving, the spread of particularly small-sized automatic printing machines each incorporated with a desensitizing treatment system is remarkable, also in the offset master by an electrophotographic system, shortening of time to printing plate making is carried out, and there are circumstances which have to meet quickening of the desensitizing treatment time and prolonging the life of the treatment liquid.
Furthermore, in the system of an electrophotographic lithographic master plate, a digital exposing system is proposed, in addition to the conventional process images mainly composed of line images and letters, the preparation of a maser plate having highly minute images of a middle tone, screen tint, etc., becomes easy, and in a printing process, a printing plate capable of reproducing the highly minute images on a printed matter has been demanded. However, it is difficult to meet the demand by using conventionally known desensitizing treatment liquids.